This invention relates generally to nuclear reactor water measurement systems, and more particularly to laser induced fluorescence water measurement systems for nuclear reactors.
A typical boiling water reactor (BWR) includes a pressure vessel containing a nuclear fuel core immersed in circulating coolant, i.e., water, which removes heat from the nuclear fuel. The water is boiled to generate steam for driving a steam turbine-generator for generating electric power. The steam is then condensed and the water is returned to the pressure vessel in a closed loop system. Piping circuits carry steam to the turbines and carry recirculated water or feedwater back to the pressure vessel that contains the nuclear fuel.
The BWR includes several conventional closed-loop control systems that control various individual operations of the BWR in response to demands. For example a control rod drive control system (CRDCS) controls the position of the control rods within the reactor core and thereby controls the rod density within the core which determines the reactivity therein, and which in turn determines the output power of the reactor core. A recirculation flow control system (RFCS) controls core flow rate, which changes the steam/water relationship in the core and can be used to change the output power of the reactor core. These two control systems work in conjunction with each other to control, at any given point in time, the output power of the reactor core. A turbine control system (TCS) controls steam flow from the BWR to the turbine based on pressure regulation or load demand.
Boiling water nuclear reactors also contain a plurality of systems to monitor the workings of the reactor. One important system is the hydrogen water chemistry system used to mitigate stress corrosion in the reactor pressure vessel. During the process of converting water to steam in the reactor, a portion of the water may be broken down into hydrogen and oxygen (2H2Oxe2x86x922H2+O2). The build-up of this dissolved hydrogen and oxygen is undesirable because it may contribute to the onset and acceleration of stress corrosion cracking of stainless steel piping and components in the reactor pressure vessel. The addition of hydrogen to the feed water causes a reduction in dissolved oxygen within the reactor internals and recirculation piping, and lowers the radiolytic production of hydrogen and oxygen in the vessel core region. To ensure that the hydrogen added to the feedwater is properly combined with oxygen to produce water, oxygen is added to the off-gas system upstream of the recombiners.
To ensure stoichiometric balance of hydrogen and oxygen is maintained, hydrogen water chemistry systems typically include chemical analyzers to monitor the level of dissolved hydrogen, oxygen, and other chemical species that directly or indirectly affect the corrosion potential of the reactor water. Typically, iron oxide, platinum, and stainless steel electrochemical corrosion potential (ECP) sensors are used to directly measure the corrosion potential of the water.
An iron oxide ECP sensor is used to measure corrosion potential when lower concentrations of hydrogen and higher concentrations of oxygen are in the reactor water. The sensor includes a zirconia crucible that acts as an oxygen ion transport membrane. This crucible is brazed to a metal sleeve which is in turn welded to a stainless steel mineral insulated (MI) cable. The cable carries the signal generated by the sensor to an electronic processor which then generates a readout. Inside of the zirconia crucible, a mixture of iron and iron oxide powder is compacted around an iron center wire that is connected to the center wire of the MI cable. The potential accross the wall of the zirconia crucible (membrane) changes with the changing corrosion potential which is due to changes in oxygen and hydrogen concentrations among others.
Platinum and stainless steel ECP sensors are used to measure corrosion potential when there are higher concentrations of hydrogen in the reactor water. These ECP sensors include either a platinum or a stainless steel cap that has been brazed onto a zirconia ceramic. The stainless steel cap is sometimes coated with noble metals to simulate the corrosion potential at the surface of a reactor component that has been coated with noble metals. The ceramic is brazed to a metal sleeve that is in turn welded to a stainless steel MI cable. When the caps are brazed to the ceramic, the center wire from the MI cable is also brazed to the cap so that there is electrical continuity between the cap and the center wire. The potential at the surface of the electrode cap changes with the changing corrosion potential which is due to changes in oxygen and hydrogen concentrations among others.
Because of the harsh environment in a nuclear reactor, for example high temperatures, high radiation, and immersion in water, the known iron oxide, platinum, and stainless steel ECP sensors have a high rate of failure. Because the location of many of the ECP sensors in a reactor are inaccessible during plant operation, a sensor failure results in the inability to collect data until the next scheduled plant maintenance shut down, when the sensor is replaced.
Further, the measurement obtained with known iron oxide, platinum, and stainless steel ECP sensors is a raw voltage representing the hydrogen and oxygen concentrations. These voltage measurements must be processed through a data acquisition system where calculations are performed to obtain corrosion potentials corrected to the hydrogen electrode scale. These corrected corrosion potentials are then used to estimate the electrochemical corrosion potential of the exposed material surfaces in various parts of the reactor. Because the calculations are based on a specific water flow rate and a particular flow profile, variations in the actual flow rate or flow profile of the reactor water through the sensor are not taken into account which may result in inaccurate corrosion potential data.
In one aspect, a water monitoring system for a nuclear reactor is provided. The reactor includes a pressure vessel, a core positioned in the pressure vessel, and a water distribution system having a water sampling port. The water monitoring system includes a fiber optic cable having a first end and a second end, with the first end configured to optically couple to the reactor cooling water distribution system, and at least one laser light source optically coupled to the second end of the fiber optic cable. The water monitoring system also includes a spectrophotometer optically coupled to the second end of the fiber optic cable.
In another aspect, a nuclear reactor is provided that includes a reactor pressure vessel, a core positioned in the pressure vessel, a water distribution system, and a water monitoring system. The water monitoring system includes a fiber optic cable having a first end and a second end, with the first end configured to optically couple to the reactor cooling water distribution system, and at least one laser light source optically coupled to the second end of the fiber optic cable. The water monitoring system also includes a spectrophotometer optically coupled to the second end of the fiber optic cable.
In another aspect, a method of monitoring the water circulating in a nuclear reactor is provided. The reactor includes a pressure vessel, a core positioned in the pressure vessel, and a water distribution system that includes a water sampling port. The method includes coupling a water monitoring system to the water distribution system of the nuclear reactor; and measuring the concentration of predetermined chemical species in the reactor water with the water monitoring system. The water monitoring system includes a fiber optic cable having a first end and a second end, with the first end configured to optically couple to the reactor cooling water distribution system, and at least one laser light source optically coupled to the second end of the fiber optic cable. The water monitoring system also includes a spectrophotometer optically coupled to the second end of the fiber optic cable.